Interface analysis and intrinsic thermal stability of Moo(x) based hole-selective contacts for silicon heterojunction solar cells

Abstract

A possible research path to increase the photo-generated current in silicon heterojunction (SHJ) solar cells is to replace doped layers on the front-side of the cell, which result in significant parasitic light absorption losses. MoOx is one candidate to replace the p-doped a-Si:H layer in such devices, although it is claimed to be relatively unstable to thermal treatments. We found that a MoOx film with a thickness of 6 nm is sufficient to achieve a J(SC) of 36 mA/cm(2), which is 0.5 mA/cm(2) on average higher than that of our classical SHJ reference cell. We also established a contact sintering condition for printed Ag at 160 degrees C after MoOx deposition, without degrading the cell performance. The champion MoOx-contacted cell yielded V-OC of 724 mV and FF of 74.1%, resulting in an efficiency of 19.3%. From a detailed analysis of the interfaces of the hole contact, an interfacial a-SiOx of 1.6-2 nm was observed between a-Si:H and MOO irrespective of the MoOx thickness (6-10 nm) before and after contact sinter annealing at 160 degrees C. We postulate that this a-SiOx layer acts as an interfacial dipole layer and also increases the contact resistivity at this contact. The intrinsic stability of the optimised MoOx-contacted cell is studied using a one-cell mini-module under standard damp-heat testing (85 degrees C/85% humidity/1000 h). More than 97 %(rel) of the original efficiency is maintained after 1010 h of testing, which is comparable to the behavior observed in a classical SHJ reference one-cell mini-module that was similarly tested.